Low temperatures– At very low temperatures, as that associated with liquid helium (boiling temperature 4.23 K that is -268.8\(^\circ \) C, close to absolute zero temperature), current in many metals, in particular in bad conductors like lead, displaying a rather strong electron-phonon coupling, is carried not by single electrons, but by weakly bound, very extended, strongly overlapping pairs of electrons, known as Cooper pairs [1]}. The role of phonons, mainly of quantal zero point fluctuation origin, is now that to act as designed glue of Cooper pairs by being exchanged between the corresponding electron partners moving in time reversal states, and not as scattering centersWithin this context it is of note that, the difference with the finite temperature phenomena is a fundamental one, namely the invasion of the large scale world by quantum mechanics. The two salient pictures of the low temperature world, the vanishing entropy and the dominance of the zero point energy are its direct consequences [2]}.. Thus conduction of current without resistance, namely superconductivity [3]}, [4]}. In a superconductor there is no resistance because all Cooper pairs carry the same phase \(\phi \) , being collectively in the same state, the coherent \(|BCS(\phi )\rangle \)  state [5]}. Once one gives a momentum to the center of mass of them and starts a supercurrent, to get one Cooper pair or one electron partner –that is, break a Cooper pair– away from what all others are doing is very hard. It implies an exchange of energy of about twice the pairing gapIn other words, an energy of the order of \(T_c\) , where \(T_c\)  is the critical temperature at which the phase transition between the normal (N) and superconducting (S) phases takes place. In the case of lead, \(T_c\approx 7.19\)  K (0.62 meV). (e.g. \(\Delta =1.4\)  meV for lead), phenomenon known as Off-Diagonal Long-Range Order (ODLRO) [6]}, [7]}, [8]}.
